Storm Sewer Lift Station - The Wilds South

About this Project

The Wilds South area, is a master planned residential development (“Property”) that is ongoing, and consists of the entire area south of 52nd Avenue W. within the City of West Fargo limits. The improvements consist of a stormwater lift station, force mains and associated site improvements to service the stormwater runoff on the property.

Historically, stormwater runoff for the property has been conveyed via sheet flow or fields swales to ditches along the east and west edges of the property and drains to the ditch along 52nd Avenue to the north of the property. From there, it drains to the Diversion when there are low flow conditions. When the Diversion is at higher flow conditions, the 52nd Avenue ditch is pumped up and over the levee into the Diversion using a Power Take Off (“PTO”) driven pump.

There are also overflow storm sewer connections into the developments system to the north of 52nd Avenue. The developed portions of the property contain a gravity storm sewer system that flows into existing regional retention ponds. The expansion of this gravity storm sewer is not part of the improvements and are planned to be installed as part of concurrent and future improvement districts.

The retention for the property consists of a series of existing regional retention ponds with the potential for additional retention ponds as part of future improvement districts. The most downstream retention pond of this system is adjacent to the proposed storm sewer lift station.

Furthermore, the storm sewer and retention pond systems within the property are at a lower elevation than the Diversion. Due to this, in addition to operating the PTO pumps, stormwater pumping measures have occurred by the contractor(s) as part of the ongoing City projects on the property to manage the current stormwater.

The existing PTO pump and temporary pumping measures are inadequate for a long term solution and are undersized for the volume of stormwater runoff generated by the proposed residential development on the property. The master plan of the development anticipated a stormwater lift station to raise the water over the levee and into the Diversion.

The existing PTO pump and temporary pumping measures are inadequate for a long term solution and are undersized for the volume of stormwater runoff generated by the proposed residential development on the property. The master plan of the development anticipated a stormwater lift station to raise the water over the levee and into the Diversion.

Based on the City’s Stormwater Retention Requirements Table dated March 2, 2006, the allowable release rate for 160 acres is a 60 inch pipe with a slope of 0.05%. Using Manning’s equation and an n value of 0.012, this equates to a flow rate of approximately 68 cfs. This equates to approximately 0.425 cfs per acre.

68 cfs ÷ 160 acres = 0.425 cfs per acre

The Property has an area of approximately 260 acres. Using the value of 0.425 cfs per acre results in an interpolated allowable release of approximately 110 cfs.

260 acres × 0.425 cfs per acre = 110.5 cfs

The proposed Improvements consist of installing a permanent storm sewer lift station at this capacity while meeting the current City design standards. As part of the master planning, a preliminary hydrologic stormwater model was developed in September 2017 using AutoCAD Storm and Sanitary Analysis (SSA) software. Ponds were designed based off this model to fit into the master plan. The model confirmed that this flow rate (110 cfs) and planned retention ponds are adequate to handle the 100-year storm of 3.74 inches.1